In-situ BWR and PWR CRUD flake analysis method and tool

ABSTRACT

The invention provides a method and tool to perform an analysis of CRUD on a nuclear fuel rod. The method recites providing a nuclear fuel rod with a layer of CRUD on an exterior of the fuel rod, scraping the CRUD from the fuel rod with a CRUD scraping tool and collecting CRUD flakes from the CRUD scraping tool. The method also provides for sorting the CRUD flakes into particle fractions, and analyzing the CRUD with a scanning electron microscope, wherein the scraping tool has a blade with a rigidity that is matched to an anticipated CRUD deposit shear strength.

FIELD OF THE INVENTION

The current invention relates to a CRUD analysis method and tool. Morespecifically, the current invention provides a CRUD analysis methodologyand a scraping tool for nuclear fuel rods to determine physicalproperties of CRUD deposited on an exterior of a nuclear fuel rod.

BACKGROUND INFORMATION

Nuclear reactors, such as pressurized water reactors and boiling waterreactors, generate nuclear power through the use of nuclear fuelassemblies housed in a reactor core. The fuel assemblies are comprisedof elongated hollow metallic tubular fuel rods that contain pellets ofenriched uranium dioxide material. The hollow metallic fuel tubularrods, commonly known in the nuclear industry as cladding, prevent theescape of materials, such as uranium dioxide and fission gasses, fromthe interior of the fuel rod. The cladding is generally configured froman alloy of differing metals including zirconium. These alloys are usedprincipally because of the low neutron capture characteristic ofzirconium. As a consequence of the low neutron capture characteristic,these alloys have been used extensively in the nuclear industry as thenuclear reaction in the reactor is least hindered by such use.

Fuel rods that have been irradiated and have had the enriched uraniumdioxide content depleted due to operation of the reactor core, are oftenstored in pools of cooling water to remove decay heat. As time passes,materials can collect on an exterior surface of the zirconium basedalloys. Moreover, the zirconium alloy may warp and swell on the exteriorportion of the fuel rod, creating a further layer of material on theunderlying sound zirconium alloy substrate. The material which collectson the exterior of the zirconium based alloy is commonly known as ChalkRiver Unidentified Deposit or “CRUD.” The CRUD located on an exterior ofthe fuel rods of a fuel assembly is generally made of solid particles,agglomerated together, that can be strongly attached to the underlyingsubstrate. Due to the closeness of the CRUD to the activated uraniumdioxide material in the fuel assemblies, the CRUD is usually highlyradioactive. The CRUD on the fuel rods can become dislodged from theunderlying zirconium alloy substrate by flowing water that the fuelassemblies are immersed in. Consequently, CRUD can enter the piping ofthe water systems in the nuclear power plant and travel along thesesystems causing unintended irradiation of personnel in plant areas thatare normally not radiologically active.

Although CRUD is a non-homogenous material, CRUD has been found to begenerally made of several elemental components. The major components ofCRUD can include, for example, iron, cobalt, zinc, silica, chrome andmanganese. As nuclear plant fuel performance is influenced by CRUDdeposition during normal plant operation as well as sequence andeconomics of refueling and maintenance outages, it is necessary toanalyze fuel assemblies for the presence of CRUD to determine the natureand amount of the deposits. For example, if it is determined that thenuclear fuel rods have a highly radioactive CRUD layer that may becomeeasily dislodged, then a worker radiological concern exists where thefuel rods may be required to be cleaned. This cleaning process isusually conducted by a number of means, ultrasonically or chemicallycleaning the exterior of the fuel rods to remove the loose CRUD buildup.

To perform analysis of crud deposited on the fuel assemblies, samplesmust be taken by mechanically scraping the exterior of the fuel rods.The systems used to perform this mechanical scraping include a rigidmember that the fuel rod is pressed against, thereby shearing the looseCRUD from the rest of the nuclear fuel rod when the fuel rod is movedover the rigid member.

Devices for mechanical scraping fuel rods may be divided into twosub-classes. Manually operated devices may be used to remove the CRUDfrom the exterior surface of the nuclear fuel rods. Such devices consistof a scraping head at the end of a shaft, wherein the scraping head isused to dislodge loose CRUD from the exterior surface of the fuel rod.Automated devices may also be used to remove deposits from irradiatedfuel rods. The fuel rods are scraped by a remote control scrapingdevice, wherein the scraped sample is conveyed to an internal reservoir.

Previous devices of both sub-categories have several drawbacks thatlimit the effectiveness of the removal devices. Existing manual devicesare only used to remove materials from an outside of a fuel rod that areeasily dislodgable. CRUD deposits that are attached to the nuclear fuelrod more tenaciously are not able to be removed using existing manualtools. As a result, the manual tools used do not provide an accuraterepresentation of CRUD materials that may be present in the entire depthof the fuel rod as the sampling occurs only on an exterior subsection ofthe total CRUD deposit. Mechanized devices, however, scrape the nuclearfuel rods such that the entire CRUD deposit is removed from a portion ofthe fuel rod surface, as well as warped zirconium alloy on the externalportion of the nuclear fuel rod. Removal of any warped zirconium alloyon an external portion of the nuclear fuel rod damages the fuel rod.This necessitates an extensive engineering analysis to determine if thepressure retaining capabilities of the fuel rod have been severelycompromised. When the material constituents of the exterior of the fuelrod are sampled after using a mechanized device, zirconium alloyinappropriately removed from the rod will skew the material analysisresults. Another drawback of existing mechanized devices for removingCRUD deposits on the exterior of fuel rods is that these devices areeconomically expensive to produce and often require significantmaintenance for operation. Additionally, performing maintenanceactivities on radioactive components of the mechanized devices increasesworker radiation exposure.

There is therefore a need to provide a CRUD removal tool, which is costeffective for nuclear reactor operators.

There is also a need for a CRUD removal tool, which will limit potentialdamage to nuclear fuel rods during removal of CRUD.

There is also a need for an analysis method for CRUD deposits, whichwill adequately categorize CRUD obtained from scraped fuel rods.

SUMMARY

The objectives of the invention are achieved as described andillustrated. The invention provides a method to perform an analysis ofCRUD on a nuclear fuel rod. The method recites providing a nuclear fuelrod with a layer of CRUD on an exterior of the fuel rod, scraping theCRUD from the fuel rod with a CRUD scraping tool and collecting CRUDflakes from the CRUD scraping tool. The method also provides for sortingthe CRUD flakes into particle fractions, and analyzing the CRUD with anumber of analytical tools including a scanning electron microscope,wherein the scraping tool has a blade with a rigidity that is matched toan anticipated CRUD deposit shear strength.

The invention further provides a method to perform an analysis of a CRUDflake on a nuclear fuel rod. The method provides providing an electronbackscattered pattern apparatus to a scanning electron microscope; andotherwise actuating the scanning electron microscope apparatus todetermine a crystal system, lattice parameter of unit cells and a pointof group crystals belonging to an in-situ portion of the flake.

The invention further provides a method for analysis of a CRUD flakecross section on a nuclear fuel rod. This method recites determining amorphology of crystals of the flake, determining a size of the crystalsof the flake, correlating elemental distributions of the flake atvarious locations on the flake, wherein the distributions are obtainedwith a scanning electron microscope with attached energy dispersivespectrometer, determining for example a depletion of iron enrichment-and/or an enrichment in zinc and silicon in the crystals by theelemental distributions, and correlating the depletion of ironenrichment and the enrichment in zinc and silicon with the size and themorphology of the crystals.

The invention also provides a method for analysis of a CRUD flake crosssection on a nuclear fuel rod. The invention provides the steps ofdetermining a morphology of crystals of the flake, determining a size ofthe crystals of the flake, correlating elemental distributions of theflake at various locations on the flake, wherein the distributions areobtained with a scanning electron microscope with attached energydispersive spectrometer, determining a depletion of iron enrichment andan enrichment in zinc and silicon in the crystals by the elementaldistributions, and correlating the depletion of iron enrichment and theenrichment in zinc and silicon with the size and the morphology of thecrystals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a fuel rod scraping tool in conformance with anexample embodiment of the present invention.

FIG. 2 is a plan view of a tool housing of FIG. 1.

FIG. 3 is an elevational view of a scraper blade of the fuel rodscraping tool of FIG. 1.

FIG. 4 is a cross-sectional view of a funnel weldment of the scrapingtool of FIG. 1.

FIG. 5 is a scanning electron microscope view of a boiling water reactorCRUD flake used during porosity calibrations.

FIG. 6 is a view of an acceptable flake sample before washing.

FIG. 7 is a view of an acceptable flake sample after washing.

FIG. 8 is a potential flake sample after washing and separation intocomponents.

FIG. 9 is a view of a fuel pin face of an acceptable flake sample.

FIG. 10 is a fluid face view of an acceptable flake sample.

FIG. 11 is a SEM view of a flake sample before tilting the flakerelative to an electron beam.

FIG. 12 is a SEM view of a flake after tilting the flake relative to theelectron beam.

FIG. 13 is an elemental map SEM photomicrograph of a flake sample foranalysis.

FIG. 14 is a SEM view of a crystalline material in a failed fuel rodflake.

FIG. 15 is a SEM view of an IO side of a flake cross-section at 3000×.

FIG. 16 is a SEM view of an OD side of a flake cross-section at 5000×.

FIG. 17 is a SEM view of the lower span cross-section OD of a flake.

DETAILED DESCRIPTION

Referring to FIG. 1, a CRUD removal device 10 is illustrated. The CRUDremoval device 10 allows CRUD to be removed from an exterior of anuclear fuel rod 24 in a safe and controlled environment. The CRUDremoval device 10 allows the CRUD to be removed in a nuclear fuel poolof a nuclear facility, for example, such that loose or firmly attachedCRUD may be dislodged. The CRUD removal device 10 further allows theCRUD to be dislodged without removing warped zircalloy material from thestabilized zircalloy substrate. The device 10 may be made of materials,such as stainless steel, to hinder potential corrosion. A blade 40 isplaced in the CRUD removal device 10 such that the fuel rod 24 runs overthe blade 40 to facilitate removal of the CRUD layer. Although shown asa single blade 40, multiple blades may be used. A housing 12 defines aninterior volume 22 which allows the fuel rod 24 to be partiallycontained within the housing 12. The fuel rod 24 penetrates the housing12 through a funnel weldment 18 which is placed into a funnel weldmentopening 16 of the housing 12. The fuel rod 24 extends from a fuel rodupper end 28 to a fuel rod end 26, wherein sections of the fuel rod 24between the fuel rod upper end 28 and the fuel rod end 26 maybe passedthrough the housing 12 during scraping operations when the rod istranslated through the housing 12.

Referring to FIG. 2, the funnel weldment opening 16 and an end opening14 in the housing of 12 are further illustrated. The end opening 14 maybe configured with a rod seal such that when a fuel rod 24 is placed inthe housing 12 and the fuel rod 24 extends out of the housing 12, therod seal prevents any materials inside the housing 12 from escaping outof the housing 12 through the opening 14. Alternatively, the end opening14 may be sized to snuggly fit around a fuel rod 24, thereby preventingmaterial from escaping the housing 12. The housing 12 is configured suchthat any volume of CRUD may be confined by the components of the housing12, including multiple scrapings of several fuel rods 24. In an exampleembodiment, the housing 12 is configured such that an anticipated volumeof CRUD dislodged from the fuel rod 24 may be sufficiently housed in theinterior volume 22 defined by the housing 12.

Referring to FIG. 3, a blade 40 for a CRUD removal device 10 isillustrated. The blade 40 is provided with a cutting surface to removeand/or dislodge CRUD material from an exterior of the nuclear fuel rod24. The removed flake material can be gathered to determine the materialconstituents of the CRUD. The blade 40 has an upper surface wherein acutting top 44 extends down from a high point in a semicircular arc to acutting bottom 42. Additionally, the blade 40 extends again from thecutting bottom 42 up to another cutting top 44 completing the arc. Thearc is configured such that an exterior edge of a fuel rod 24 may beplaced on the blade 40 for removal of CRUD. The blade 40 is configuredwith sides 46 which extend down from the cutting top 44. The sides 46may be lengthened or shortened such that the blade 40 is provided withflexibility. The rigidity of the blade 40 may be chosen according toanticipated CRUD deposit sheer strength. The blade 40 is designed to notbe rigid when a fuel rod 24 is contacted on the cutting surface therebyallowing superior CRUD removal capability. The sides 46 may belengthened or shortened based upon the anticipated force to be exertedon the fuel rod 24 during scraping operations, as well as a totalthickness of the blade 40. The sides 46 may be reduced in overall widthby a width reduction 52 that allows for the required flexibility of theblade 40. The blade 40 may be attached to the housing 12 through asupport rod that extends through a support rod hole 48 in the blade 40.The size of the support rod hole 48 may be adjusted such that differingsizes of support rods may be used in conjunction with the blade 40. Thesupport rod hole 48 provides a snug connection between the support rodand the blade 40 to allow the blade 40 to be utilized for scrapingoperations without the blade 40 becoming dislodged or tilting anexcessive amount. A blade bottom 50 is placed on the housing 12 suchthat the blade bottom 50 contacts the housing 12 thereby preventingrotation of the blade 40 during scraping operations. Although shown as aflat bottom, the blade bottom 50 can be configured in any arrangementthat would fit a corresponding bottom of the housing 12.

Referring to FIG. 4, a side cross-sectional view of a funnel weldment 18is illustrated. The funnel weldment 18 is comprised of a cone 54 whichextends from an opening 58 to a exit end 20. The funnel weldment 18allows a fuel rod 24 to be inserted into the housing 12 through theopening 58 to allow for precise placement of the fuel rod 24 inside thehousing 12 during scraping operations. A seal may be placed around theweldment 18 and in the housing 12 to limit material from exiting thehousing 12. The exit end 20 may be positioned such that a guide way 60will direct the fuel rod 24 along a desired pathway in the housing 12.The guide way 60 may be angled such that the guide way 60 snuggly fitsaround the exterior periphery of the fuel rod 24. The guide way 60 mayalso be configured with an internal seal 62 to allow the fuel rod 24 tobe placed within the funnel weldment 18 such that scraped material willnot pass out of the housing 12.

The CRUD scraping tool 10 may be attached to a structure in the fuelpool such that after use, the tool 10 may be removed from the fuel poolenvironment. The housing 12 may be opened and the scraped particles maybe removed for further analysis using a Scanning Electron Microscopedescribed below. The scraping tool 10 may also have a demineralizedwater intake and outlet such that CRUD material loosened during scrapingcan be gathered in a filter housing located near or at the outlet of thehousing 12.

New Crud Flake Analysis Methodology

A new crud flake analysis methodology has also been developed for BWRand PWR crud using traditional analytical techniques, including SEM/EDS,XRD and ICP/MS. The focus of the evaluation is shifted to CRUD flakeanalysis from bulk properties to obtain a more clear understanding ofcrud morphology and spatial distribution of contaminant species. To doso all analysis are done on intact flakes.

Objectives of the CRUD flake examinations are the following:

-   -   To physically characterize the CRUD morphology;    -   To determine the presence, size and shape of crud boiling        pockets at the fuel surface;    -   To determine the presence, size and shape of steam chimneys and        capillaries;    -   To determine overall porosity/density of the deposit;    -   To provide microscopic EDS elemental analysis inside crud        boiling pockets and in other locations of crud situated at the        fuel surface;    -   To attempt to isolate the crud boiling pockets' characteristic        morphologic formations for further analysis.

These examinations included Inductively Coupled Plasma (ICP)Spectroscopy for total concentration of common crud/fuel oxide metals,microphotography, X-Ray Diffraction (XRD) to qualitatively identify themajor crystalline components in solid bulk samples, scanning electronmicroscopy (SEM) including energy dispersive spectroscopy (EDS), andporosity and density measurements. All these analyses have beenperformed up to now for PWR or BWR bulk fuel CRUD, due to the method ofsampling and the of CRUD nature on bulk samples

SEM/EDS provide qualitative and quantitative analysis, elementaldistribution, and surface topography and guides other analyses from animage at high magnification. Also gamma spectroscopy was performed todetermine specific bulk activity of crud samples that were analyzed formetals.

There are known disadvantages for all types of the bulk analysis. Forexample, in XRD: The CRUD transfer consisted of removing manually thedeposit from a portion of the filter, transferring the deposit to avessel with water methanol mixture and manually stirring the solutionwith a glass wand to suspend in the liquid. The resultant slurry wasfiltered through a 5 micron filter. The filtered solid was transferredto a glass slide and fixed for X-ray diffraction. The transfer itselfwas required because the filter material spectra interfered with thecrud material spectra.

Another known disadvantage of the present analysis method is that X-RayDiffraction (XRD) was used to qualitatively identify the majorcrystalline components in solid samples such as oxides (e.g., magnetite,hematite, alpha-iron, etc.) in bulk samples.

Another known disadvantage is that since the selection of the samplematerial to be used in the test is random, there is no assurance thatthe XRD plot obtained truly represents the totality of the sample.

Another known disadvantage of the bulk analysis methods is that thequantity of solid material collected has to be (by flake scrapingmethods) large to insure an adequate homogeneous mixing of the CRUDsample. That makes it possible that some of the species existing in theCRUD may not be contained or detected in the sample.

Another known disadvantage is that existing trace quantities may not beidentified by XRD because of the small sample size. Amorphous phases arenot determined by XRD techniques.

Another known disadvantage is that the sensitivity of XRD for a givencompound varies with a combination of such factors as density, degree ofcrystallization, particle size, and coincidence of strong lines fromother constituents and the kind and arrangement of the atoms of thecomponents. When a high fraction of iron is present (as with most CRUDdeposits), some constituents are difficult to detect because of the highmass absorption coefficient of iron. As a result, the detectionsensitivity for these constituents decrease and their limit of detection(LOD) and limit of quantification (LOQ) increase.

XRD is not generally considered a quantitative analysis technique,although relative percentages of compounds detected can be establishedand characterized as major (greater than 25%), medium (10%-25%), minor(5-10%) and trace (less than 5%) levels. In some cases, calibrationcurves can be generated using known quantities of one or more species ofinterest to estimate their percentages in samples to approximately +5%.Minimum sensitivity can be as low as about 0.1 percent.

An additional limitation of bulk techniques as applied today for XRD,SEM/EDS analyses etc. is that the samples have to be pulverized andmixed to obtain adequate spectra for analysis. As a result, importantinformation on CRUD deposit stratification is lost. Because the physicalstructure of the CRUD is as important, and likely more important, todetermining its effect on nuclear fuel performance, the loss of thisinformation is a serious limitation of previously-available analysistechniques.

The discussion above stresses again the fact that for fuel flakeanalyses, the XRD method provides only an indication of the speciesexisting in the crud. Further confirmatory work must be done throughSEM/EDS.

Changes in Preparation of Flakes for SEM/EDS Method to Adapt Itself toIn-Situ Analysis of CRUD

As part of the CRUD flake analysis, SEM/EDS techniques were modified andrefined in order to provide significant data for in-situ analysis.Standard ICP, XRD, and gamma spectroscopy techniques were used to aid ininterpreting the SEM/EDS data.

To examine the fuel flakes through in-situ method, special radiologicalprecautions are followed. Some of these samples have large β and γradiological fields. Contact with the samples was carefully monitored tocontrol exposure. All activities were conducted under stringentradiological protection methodology. Accidental mishandling of thesesamples can result in the spread of contamination and creation of hotspots. All work areas were cordoned off while handling these samples andcleaned up immediately after each operation.

The preliminary selection of a flake consisted in selecting from a largenumber of filtered scrapes identified on a given filter paper the onesthat looked larger and sturdier. Many of the large scrape samples may besimple agglomerations of amorphous substance, mostly Hematite. Whenlightly touched, these agglomerations disintegrate.

The scrapes that looked and behaved sturdier were first examined under amicroscope to see if the sample could be considered a flake, and howfree it was of loose Hematite. FIG. 6 presents a potentially acceptableflake sample, heavily covered with Hematite. If Hematite was attached tothe sample, as in the case of FIG. 6, then combinations of successivewashings and scrubbings would eventually clean the potential flakesample, as presented in FIG. 8. Note that FIGS. 6 and 7 present the samepotential flake sample before and after washing.

Successive examinations under a microscope of the potential flake sampleof FIG. 7 revealed that the flake actually consists of two flakes (seeFIG. 8). One of the samples was mounted vertically and another onehorizontally.

The next step in flake sample preparation was the optical examinationfor orientation, where a number of steps were conducted to allow thepositive identification of the ID and the OD of the flake. The stepsconsisted in taking a series of microphotographs of each side of thesample, observing significant differences and then proceeding toclose-up microphotographs. The latter examinations revealed throughcurvature and smoothness of surfaces if a bona-fide ID (fuel pin side)or OD (fluid side) flake surface had been identified. The process may beby trial and error. Accordingly some of the steps were modified based oninitial observation of the condition/type of the subject flake.

A successful flake would have both the ID of the flake (fuel pin side)and the OD side (fluid side) clearly identified and their featuresun-modified by previous flake handlings or foreign objects. FIGS. 9 and10 present a photomicrograph of an acceptable flake sample ID and ODsurface. Features on both surfaces are clearly identifiable

Modified Scanning Electron Microscopy—Energy Dispersive SpectrometerAnalyses

FIG. 10 is a fluid face view of an exemplary acceptable flake sample.FIG. 11 is a view of a flake before tilting of the flake with respect toa scanning electron microscope beam. FIG. 12 is a view of a flake aftertilting to provide a clearer view. FIG. 13 is an elemental map and SEMphoto micrograph of a flake sample. FIG. 14 is a view of a crystallinematerial in a failed fuel rod.

The modified combination Scanning Electron may be used inMicroscope-Energy Dispersive Spectrometer (SEM/EDS) the new flakeanalysis methodology described herein. It is used in conjunction withthe previously-available bulk techniques (ICP, XRD and GammaSpectroscopy) in order to extrapolate and integrate the criticalinformation of local CRUD characteristics in the assembly of the datafrom crud.

Due to the small size of fuel crud samples collected, all bulk analysesin the example embodiment (ICP, XRD and Gamma Spectroscopy) have a highdegree of uncertainty. Both the sensitivity of the techniques, as wellas the randomness of the samples collected is negatively impacted if thequantity of the CRUD collected is very small.

SEM uses a finely focused electron beam to form an image at highmagnifications (up to 20,000× in the present work) with virtuallyunlimited depth of field. The electron beam generates secondaryelectrons, backscattered electrons, and characteristic X-rays as it isscanned across the surface of the sample. The secondary andbackscattered electrons are used to form viewable images of the samplecreating very accurate surface topography images of features only a fewnanometers (nm) across. In some instances, in the present work, featuresno bigger than 200 nanometers had to be identified in the crystallinemake-up of portions of crud deposit. Function of the bulk conductivityof the CRUD. As the SEM image magnification increases, depending on thebulk conductivity of the CRUD, surface charging can occur. This has attimes limited the achievable magnification in regions of interest.

With the attachment of an energy dispersive spectrometer (EDS) to theSEM, the chemical analysis (microanalysis) of the CRUD flake sample wasperformed by measuring the energy or wavelength and intensitydistribution of X-ray signal generated by a focused electron beam on thespecimen. The advantage of using the SEM/EDS combination for small flakeanalyses was that very precise and accurate chemical analyses (relativeerror 1-2%) can be obtained from areas of the solid no larger than 0.5-3micrometer diameter. This is important for the credibility of creviceanalyses performed on CRUD flakes to evaluate specific local conditions.

Yet another attachment to the SEM, which is the electron backscatteredpattern (EBSP) apparatus and method, can be used to successfullydetermine the crystal system, lattice parameters of the unit cells andpoint accordingly towards the group of the crystals belonging to anin-situ specific portion (crevice) of the flake.

The methodology of SEM/EDS analysis for each of the individual flakeswas modified by special flake specimen preparation (identification ofadequate sample, cleaning of tramp material, mounting of the delicatespecimens, and careful exposure of “pristine” surfaces forinvestigation). A summary of the developed methodology analysis follows(the preparation being described previously).

SEM Examination of Fuel and Fluid Side of Flake

The fuel side and the fluid side of the flake were examined at lowmagnification to find a reasonably flat area. The area was magnified andan EDS and elemental map at magnifications of 100-1000× were taken.Specific features of the area were identified in each case, whichappeared as a crevice, crack or hole on that flat area. Highmagnification micrographs of features (showing the morphology) whichhave the largest volume (either large diameter and/or depth of thefeature) were taken. An EDS and elemental map of two of the largestfeatures were performed. The flatness near the observed feature wasanalyzed such as to select enough flat surfaces to form a referenceplane. Stereoscopic images were taken when possible for furtherevaluation.

SEM Examination of the Edge of Flake

The correctness and accuracy of the SEM/EDS examination of the edge of aflake depended on whether a good clean break edge was available or couldbe created.

The edge resulting from splitting the flake at a weak point is rich infeatures, so, in the example provided, the flake has to be mounted suchas to reflect its unique characteristics. In many cases successivemountings were performed until the flake was successfully positioned.The edge area of the flake was examined at low magnifications for asmooth/flat area. EDS and elemental map at magnifications of 500-1000where taken. Specific features of the edge of the flake such ascrevices, steam channels, and capillaries were identified. Highmagnification micrographs of features (showing the morphology) havingthe largest volume were taken to support future development of BWR fuelcrud model development.

The principle of method for cross-section analysis consists incorrelating the SEM/EDS elemental distributions at various locations ofthe cross-section of the CRUD to follow the depletion in Fe andenrichment in Zn and Si in particular, i.e., individual, crystals and tocorrelate this chemical information with the size and morphology of thecrystals, as well as with their crystallography.

In order to more accurately analyze flakes, a method of measuring thepercent void area as a measure of porosity was developed.

In the present example, it was quite difficult to obtain good SEM datafrom the fuel pin side of the flake due to its slope. The flake,however, was tilted to get the best view of fuel pin side (FIG. 12). Asa remedy the sample is tilted until the best view is obtained.

Yet another application of tilting is to correlate SEM images ofindividual crystals and EDXS spectra from the same crystals using asmall probe, i.e., in the spot mode, so the chemistry can be correlatedwith the size/morphology of individual crystals. This may requiretilting the crystals in the SEM to investigate crystal morphology, i.e.,to see if they have square or hexagonal shapes and therefore 4-fold(cubic or tetragonal crystal systems) or 6-fold (hexagonal crystalsystems) rotation axes.

Tilting of a crud samples of very small size (microns in diameter) canbe done using special props or simple glue for example.

There are several advantages of in-situ method to identify phenomena notseen before using classical methods. These include:

a) Identifying crystalline structures at their place of formation inCRUD and comparing them with catalog formation. Advanced localizedcrystal analysis using SAED or other methods of small area electrondiffraction techniques may also be performed.

As an example, copper can be identified in-situ in a separateagglomeration within a crystal matrix containing only zinc and silica.This observation is possible through elemental mapping andphotomicrographs of discrete CRUD flakes and EDS analysis at 20,000× ofthe same area.

The morphology of the crystals can be further compared with knowncrystals. In our example plan/micrograph a crystal formation with themineral crystal Willemite may be shown. Willemite is a neosilicate (ZincSilicate Zn₂SiO₄) formed by linking individual silica tetrahedra throughpositively charged zinc ions in elongated hexagonal prisms crystals. Theidentification of these types of crystal formation are important inevaluating nuclear plant water chemistry as it affects CRUD formationand transformation during nuclear plant operation.

-   -   Although the visual resemblance of crystals is striking,        definitive crystal identification needs the exact crystalline        phases of the crystal. This may be done in combination with        individual crystal elemental analysis, preformed through small        area electron diffraction (SAED). The advantage in analyzing        flakes is that it allows in-situ SAED, which can lead to a much        better understanding of both crevice and steam chimney        phenomena.        b) Observing and understanding the coexistence between the        minority crystalline structure and the majority compact        structure as happens in most CRUD deposits.

The OD side of the flake cross section locations indicates such a case,explaining how silica crystals or silicates incorporates in CRUDdeposit.

The flake edge OD and ID locations are where high magnificationexaminations are most beneficial in identifying internal CRUDcharacteristics. Location #1 in FIG. 15 is the 3000× microphotograph ofthe upper part of an fuel pin side crevice and the beginning part of asteam chimney. Location #2 in FIG. 15 is an intermediary steam chambersomewhere on a steam chimney. This type of analysis resolution allows anassessment of the elements contributing to deposition on the walls ofsuch an intermediary steam chamber. All three locations in FIG. 16 (a5000× microphotograph of the fluid side of a CRUD flake cross-section)potentially explain the mechanism of silica crystal incorporation intothe deposit.

There are some impediments connected with the relative “large” size ofthe flake and subsequently the relatively large radioactivity obtainedfrom the flake, which affects the EDS and microphotograph quality athigh magnification. The strong gamma emissions by the flake samples can“blind” the SEM/EDS detector and/or skew the EDS results. A method tocompensate for this effect was developed and is given below.

The X-ray “glow” in-situ treatment of the sample was recorded. From thisinformation, it was apparent that the high intensity X-ray emission dueto decay of ⁵⁵Fe and ⁶⁵Zn (among other species) of CRUD flakes willslightly bias the EDS data evaluation. In this case, the results of theEDS spectra have been adjusted in order to present a realisticassessment of the flake composition. Other elements can show such a“glow” as to preclude their analysis.

c) Observation of unique phenomena—insertion of structures from bulkinto the loose surface crud.

FIG. 17 presents a microphotograph of location #1 on the OD side of aCRUD flake. Here two crystals of chain silicates, needle-shapedstructures approximately 0.2 μm in diameter and most likely formed inthe bulk water, are seen inserted by the flow in the loose surface crud.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made there andtoo without departing from the broader spirit and scope of the inventionas set forth in the appended claims. The specification and drawings areaccordingly to be regarded as in and illustrative rather than arestricted sense.

Flake Porosity and Density

For flaked particles, there is a correlation between the values ofdensity and porosity, wherein if one of the values of density orporosity is known, the other value may be calculated. When both densityand porosity are measured, an error correlation for the measuringmethodologies may also be determined. For flakes from nuclear fuel rods,however, density measurements of relatively small CRUD particles cannotbe accomplished due to the small number of particles obtainable fromeach rod. Additionally, traditional porosity measurement methods cannotbe used due to the extremely small volume for the flake samples. Thepresent invention alleviates these limitations because flake porosity isestimated based on a Scanning Electron Microscope (SEM) image of aflake. The method of calculating porosity using an SEM is accomplishedby initially calibrating visual data obtained by the SEM on referencematerial to establish a density measurement for of the referencematerial. Errors between measured and calculated valves are expressed asa percentage difference between average porosity determined from directdensity measurements, via “solid density”, and a calculated averageporosity that is obtained by averaging SEM image determined porositiesin various locations of the flake cross section. “Solid density” isdefined as a sample density which used our assumed atomic composition.This allows the establishment of a reference point, with elementspresent in a sample sharing 100% of the volume of that sample.

A typical SEM image is produced when the SEM ion beam is rastered acrossa sample surface. As the ion beam interacts with the sample surface, anemission of secondary electrons occurs from the surface. These emittedelectrons are detected by the SEM that has a detector to record theseelectron emissions. When the electrons strike a topographically elevatedportion of a CRUD sample, a greater percentage of the ion beam energy istransferred to the emitted secondary electrons. These secondaryelectrons carrying more energy are detected by the SEM as theseelectrons give a stronger secondary electron signal. The SEM theninterprets the elevated energy of these secondary electrons as atopographically high area.

As the SEM electron beam is rastered across the CRUD sample, an imagecontaining many discrete pixels, is produced. Each pixel corresponds toone discrete location at which the ion beam was focused and thebrightness of each pixel is proportional to the topographical height ofthe sample at that location. The lighter (i.e., white) pixels representthe “high” elevations, while the darker pixels represent the “low”elevations.

Calibration Method

SEM micrograph images have a distribution of high (light) and low (dark)areas. SEM micrograph images, however, may be skewed toward a lightershade (i.e., there are no black areas or a lower shade). This skewing or“offset” of SEM images varies from image to image.

In order to normalize the SEM image contrast (distribution betweendarkest and lightest points), the SEM image is adjusted by setting thedarkest points to be pure black and the lightest points to be purewhite. This adjustment “calibrates” the contrast of the image to theparticular topographical distribution of the sample. Once an image hasbeen properly calibrated, it is possible to use the calibrated image forporosity measurements. In order to perform porosity measurements usingSEM images, it is necessary to calibrate the image data using previouslymeasured density values.

To make a direct (empirical) density measurement, at least one CRUDflake is required to have a sufficient mass for weighing. After weighingthe flake, an area of a representative side of the at least one flake iscalculated from information provided by the SEM image. An edge image ofthe flake is also obtained, allowing the calculation of a uniformthickness for the flake. An average width of the flake is then estimatedusing this data. Assuming that the flake is uniform in thickness acrossall its area, the volume of the flake is next calculated. Next, thedensity of the flake is calculated as follows:

MeasuredDensity=Weight/Volume

To determine the porosity, the “solid density” of the flake iscalculated. The calculation method for the “solid density” utilizes theatomic composition provided by the SEM analysis. Using the atomiccomposition data, the normalized molar composition is calculated basedon an assumed compound mix. Additionally, an average densitycontribution of each element and total average solid density iscalculated.

The relationship between solid density and porosity is:

$\begin{matrix}{{Porosity} = {\left( {1 - \frac{MeasuredDensity}{SolidDensity}} \right)*100}} \\{= {\frac{VoidVolumeDensity}{SolidDensity}*{100\lbrack\%\rbrack}}}\end{matrix}$

For an infinitesimally small unit depth, porosity can be expressed as afunction of surfaces as:

${Porosity} = {\frac{VoidSurface}{TotalSurface}*{100\lbrack\%\rbrack}}$

The next step of the porosity measurement method is to establish avirtual plane, wherein cutting the flake SEM image would result in aporosity expressed as a function of void surface of the total normalizedSEM surface for the above determined porosity. A “baseline” grayscalevalue for all pixels is determined, where values less than or equal tothe baseline grayscale value account for an approximate percentageporosity of the total image pixels of the SEM image.

In the exemplary embodiment as provided in FIG. 5, a flake with aporosity of 47.77% is illustrated, the baseline grayscale of the SEMimage is measured as a 92 value, for which 47.77% of the pixels aredarker than or equal to the grayscale value. The maximum grayscale valueis, for example, 255. For a material with high porosity, the materialwill have a larger number of points close to a “0” position, because thematerial has many more crevices (values under the average grayscalevalue 92). In the case of a less porous material, the number of pointsclose to 0 would be reduced, and the number of points close to “255”position larger.

Measurement Results

Using this method, the porosity of other CRUD flakes may be estimated.In sample analysis performed, it has been determined that CRUD may bemuch denser in failed regions of nuclear fuel rods that unfailedregions. It has also been determined that there is an error band for theaverage computed data compared with the average measured data ofapproximately +/−14.5%.

The following principle steps are performed using the methodologypresented above:

-   1. Crop the SEM image to eliminate any labels that may affect the    results.-   2. Adjust the contrast of the SEM image such that the darkest points    are pure black and the lightest points are pure white.-   3. Determine the density of at least one flake sample through    empirical measurement and through the “solid density” concept for    the porosity of that sample.-   4. Determine for that sample the percent of pixels that have a    grayscale value equivalent to the porosity as determined from    empirical density data.-   5. Apply the grayscale equivalent to the rest of flake sample cross    sections.-   6. The percentile of pixels that are darker or equal to that    grayscale value is equal to the approximate porosity of the sample.-   7. From porosity values, the density of a flake results wherein the    porosity and the solid density are used to determine the density of    the flake sample.

Table 1 presents an example calculation of a solid density for ahypothetical CRUD flake sample.

TABLE 1 Avg Theo- Density Normal- retical Contri- Assumed ized Densitybution Element Atom % Compound Mol % Mol % (g/cc) (g/cc) Mg 2.58 MgSiO₃2.58 3.3 3.19 0.10 Al 2.4 Al₂SiO₅ 1.2 1.5 3.145 0.05 Si 19.39 SiO₂ 13.717.3 2.334 0.40 P 0.86 P 0.86 1.1 1.823 0.02 Ca 0.29 CaSiO₃ 0.29 0.42.92 0.01 Cr 1.23 CrO₂ 1.23 1.6 4.89 0.08 Mn 1.62 MnSiO₃ 1.62 2.0 3.480.07 Fe 27.75 Fe₂O₃ 13.875 17.5 5.25 0.92 Cu 12.85 CuO 12.85 16.2 6.311.02 Zn 31.04 ZnO 31.04 39.2 5.6 2.19 SUM 100.01 79.245 100.0 Average4.87 Density (g/cc) Note: Calculation performed for Exemplary Flakeusing EDS atomic % values

TABLE 2 Table 2 presents an idealized porosity calculation for ahypothetical CRUD flake sample provided in Table 1. Flake A Flake BFlake D (Sample (Sample (Sample #7) #17) #18) Flake Average N/A 47 N/AMeasured Porosity (%) Flake Average 32.5 55 58 Computed Porosity (%)Flake Edge OD 41 48 Side Measured Porosity (%) Flake Edge 40 44 43Middle Side Measured Porosity (%) Flake Edge ID 25 80 84 Side MeasuredPorosity (%) * An error band of +/− 14.5% should be considered betweenaverage measured and average computed porosity. The measured porosityhas been obtained from empirically measured flake density.

Table 3 presents an exemplary calculation for denoting based on SEMImages.

TABLE 3 Flake A Flake B Flake D (Sample (Sample (Sample #7) #17) #18)Flake N/A 2.56 N/A Measured Density (g/cc) Flake Average 3.79 2.19 2.00Computed Density (g/cc) Flake Edge OD 2.87 2.50 Side Computed Density(g/cc) Flake Edge 3.37 2.73 2.74 Middle Side Density (g/cc) Flake EdgeID 4.21 0.97 0.77 Side Computed Density (g/cc) * An error band of +/−14.5% should be considered between flake measured and flake averagecomputed density.

1-11. (canceled)
 12. A method to perform an analysis of a CRUD flake ona nuclear fuel rod, comprising: providing an electron backscatteredpattern apparatus to a scanning electron microscope; and actuating thescanning electron microscope to determine a crystal system, latticeparameter of unit cells and a point of group crystals belonging to anin-situ portion of the flake. 13-23. (canceled)